Feedback control and beamsteering in solid state laser arrays

Generation of a single beam of high power, coherent radiation from semiconductor injection laser array stripes is not currently possible due to the unstable nature of these arrays. When coupling between the individual elements is strong, a stable phase locked state does not exist. Coherent emission is thus impossible.;A modified form of previously developed feedback laws for stabilizing semiconductor injection laser arrays about the lowest order supermode, are experimentally implemented in a two element, solid state Nd+3: YAG laser array. The model equations describing the array are the same in both systems except that the antiguiding coefficient is zero in the Nd +3:YAG system. Furthermore, a stable phase locked state already exists in the Nd+3:YAG system though it is the antisymmetric supermode which produces a two lobed far field intensity profile. The feedback laws are implemented in an attempt to force the array to the lowest order supermode (symmetric) in which the far field intensity profile is a single lobe.;The modified feedback implementation in the two element array is accomplished by modulating both pump beams identically with the detected and amplified output of a single element. Apertured detectors simultaneously sample the on-axis and off-axis far field intensity profile for the presence of each supermode as the feedback gain is increased. With increasing feedback gain, the time for the symmetric supermode to turn on decreases. At the highest feedback gains, the symmetric mode intensity is observed to be larger than the antisymmetric mode intensity from the onset of lasing.;Beam steering through phased array techniques is also investigated. It is shown theoretically that small amplitude, sinusoidal modulation of the injection current of one element in a feedback stabilized semiconductor injection laser array causes sinudoidal oscillations in the phase difference between the elements of the array. The oscillations depend on the modulation frequency and amplitude of the pump. Steering angles of a few degrees are predicted depending on element spacing and output wavelength. Experimental results from the Nd+3:YAG system show that the phase difference is indeed affected in a periodic manner through small amplitude modulation of a single pump beam.